Deprecate multi_span, strided_span, etc

[Jordan Maples (JordanMaples)]

A number of features exist in the GSL that are not a part of the Core Guidelines.

This PR marks all classes and structs within the multi_span header as deprecated.

[Jack Diver (nitronoid)]

This is sad.
The multi-span implementation provided here is the best one I've found and suits all my needs out of the box.
Would you please consider supporting it separately instead of completely abandoning the idea?

[Jordan Maples (JordanMaples)]

Maintainers' call: Approving this. If you require this, I recommend creating a copy for yourself as we will no longer be maintaining this portion going forward.

[Jack Diver (nitronoid)]

To anyone who needs it, I have created a standalone repo for multi_span here: https://github.com/nitronoid/multi_span

Jordan Maples (@JordanMaples) Could you let me know whether I have done the licensing correctly?

[Jordan Maples (JordanMaples)]

Jack Diver (@nitronoid) It looks correct to me, however I'm not an expert with licensing.
Gabriel Dos Reis (@gdr-at-ms) or Herb Sutter (@hsutter) would either of you mind looking at Jack Diver (@nitronoid)'s licensing on the multi_span mirror (https://github.com/nitronoid/multi_span)?